Ferroelectric and piezoelectric materials have garnered considerable interest due to their numerous applications in diverse commercial markets, e.g., medical imaging devices, next generation inkjet printer heads, precision positioning stages for microscopes, fuel injectors in diesel engines and memory devices. The macroscopic properties of ferroelectric and piezoelectric materials that make them attractive for such technologies can be more fully understood and improved through detailed knowledge of their domain structures at the nanoscale and mesoscale levels.
Scanning probe microscopes (SPMs) have been employed for studying ferroelectric and piezoelectric materials. To aid in understanding the structures of these materials, one well-established microscopy technique that has been applied extensively to ferroelectric materials is Piezoresponse Force Microscopy (PFM), a scanning probe technique that enables the visualization and manipulation of ferroelectric domain structures at the nanoscale. In PFM, a voltage is applied to the material and the inverse piezoelectric effect is employed to detect a motion of the sample surface. More particularly, PFM uses an external AC voltage to modulate the strain induced by the inverse piezoelectric effect while monitoring a resulting deformation wave in terms of both amplitude and phase.
PFM requires a lock-in amplifier, which enhances the inherently small vibration signal of the sample. The lock-in amplifier is needed to detect the signal due to the small surface deformations in the material. However, one of the major drawbacks of PFM is that the speed of data acquisition is limited by the resonance frequency of the cantilever of the PFM test apparatus and the time constant of the lock-in amplifier. PFM images are seldom acquired at scan frequencies higher than 10 Hz over a scan length of 10 μm. To acquire images at relatively higher frequencies, advanced lock-in equipment and specialized, expensive hardware are needed (see e.g., US 2008/0192528 to Siegert et al.). The scan speed limitations of PFM due to the resonance frequencies of the cantilever and the time constant of the lock-in amplifier restrict the application of PFM to investigate dynamic properties of piezoelectric and ferroelectric materials, and hinder the efficiency of PFM techniques. Further, the excitation voltage necessary for PFM has the potential to influence dynamic behavior in ferroelectric films.